I have a few friends who are engineers from Georgia Tech and when I asked them about the video, they told me it's true, the sound that Mizuno irons are making are due to them providing a softer feel.
I hear a lot of people ask about the softest feeling irons. A few years ago there was a study taking golfers using forged irons versus cast irons in a blind test and the results were that the golfers could not tell the difference in the softness in the feel.
However, according to Scratch Golf founder Ari Techner, what the study failed to show was that it's not about whether the club is forged or not, it's about what type of steel is being used.
I've tried out quite a few clubs and here's some of the softest feeling irons I've hit:
I carry the 52-degree Gap Wedge of the Nickent ARC series (it's bent down to 50 degrees). From the Nickent Web site (http://www.nickentgolf.com/)
A true forged iron for the player looking for a classic iron updated with the most current technology. The blade design has Accelerated Rebound Core (ARC) technology, which creates an internal cavity that allows the club to have the effect of perimeter weighting. It works like a corked bat. This results in an iron with the look and feel of a true muscle back with the playability and ease of use of a modern cavity back.
A high-rebound elastomer is positioned directly behind the impact zone for a lively, solid reaction off the face while elimination vibration.
I really like the Gap Wedge I have. The rumors have been swirling that Nickent could be going out of business soon and I like the Gap Wedge so much that if the price of the ARC blades goes down drastically, I may purchase a set soon even though I love my Mizunos. But these clubs do have a great, soft feel. The Nickent ARC iron set goes for about $799.
Miura irons (http://www.miuragolf.com/) are known for their soft feel. I hit some the other day quite extensively. I thought they had a very nice feel and if you're looking for an old-school type of blade designs, this is certainly your club. However, I actually thought my Mizuno's were slightly softer. Miura irons go for over $1,200 a set.
Scratch Golf (http://www.scratchgolf.com/) irons are forged from 1018 Soft Carbon Steel, which according to Scratch Golf is the softest and most responsive steel in golf. I've tried their JLM Wedge, but I didn't really care for it. But when it comes to their other wedges and irons, they've received rave reviews, particularly for their soft feel. Scratch Golf iron sets go for about $1,200.
KBS Tour shafts have become increasingly popular on all of the major tours. I have yet to hit some of the KBS Tour shafts, but these shafts have received nothing but rave reviews. I've been told they are almost like a cross between the True Temper Dynamic Gold steel shafts and the Rifle Royal Precision steel shafts, only better. KBS Tour shafts go for about $25-$30 a shaft.
Iomic grips (www.iomicusa.com) have become increasingly popular on the Tour. In fact, Verizon winner Brian Gay uses them on his irons. They are made out of elastomer instead of rubber and they are water resistant as well as help make the shots feel 'softer.' They go for about $30 a grip.